The present invention relates to new rubber compositions having a base of precipitated silica intended for the manufacture of road tires of improved resistance to rolling.
Since savings of fuel and the need for protecting the environment have become a priority, it is desirable to produce polymers having good mechanical properties and as low a hysteresis as possible in order to be able to use them in the form of rubber compositions which can be employed for the manufacture of various semi-finished products entering into the creation of tire bodies such as, for instance, underlayers, calendering layers, or sidewalls or treads, and to obtain road tires (cycles, motorcycles, passenger-car tires, heavy-vehicle tires) having improved properties which in particular are of reduced resistance to rolling.
In order to achieve this object, numerous solutions have been proposed consisting, in particular, of modifying, inter alia, the nature of the diene polymers and copolymers towards the end of polymerization by means of coupling or starring or functionalizing agents. All of these solutions have been concentrated essentially on the use of modified polymers with carbon black as reinforcing filler in order to obtain a good interaction between the modified polymer and the carbon black. It is known, in general, that in order to obtain the optimal reinforcement properties conferred by a filler, the latter should be present in the elastomeric matrix in a final form which is both as finely divided as possible and distributed as uniformly as possible. Now, such conditions can be realized only to the extent that, on the one hand, the filler is of very good ability of being incorporated in the matrix upon the mixing with the elastomer and of disintegrating or deagglomerating itself, and of being dispersed uniformly in the elastomer. The use of reinforcing white fillers and particularly of silica has proven inappropriate by reason of the low level of certain properties of such compositions and therefore of certain properties of the tires using these compositions.
Furthermore, for reasons of reciprocal affinity, the silica properties have a harmful tendency, in the elastomeric matrix, of agglomerating with each other. These silica/silica interactions have the harmful result of limiting the reinforcement properties to a level which is substantially below that which would be theoretically possible to obtain if all the silica/elastomer interactions capable of being created during the mixing operation were actually obtained.
Furthermore, the use of silica raises difficulties of processing due to the silica/silica interactions which tend in raw state (before curing) to increase the consistency of the rubber compositions, and in any event to make the processing more difficult than the processing of the carbon black.
The interest in silica reinforced compositions has been recently brought up again with the publication of European Patent Application EP-A-0 501 227 which discloses a sulfur-vulcanizable rubber composition obtained by thermo-mechanical working of a conjugated diene copolymer and an aromatic vinyl compound prepared by solution polymerization with 30 parts to 150 parts by weight for 100 parts by weight of elastomer, of a special precipitated silica. The use of such a silica has, to be sure, reduced the difficulties in processing mixtures containing it, by way of majority or not, as reinforcing filler, but the processing of such rubber compositions nevertheless remains more difficult than the processing of carbon black.